24 CHEMICAL TRANSFORMATIONS 



reaction is zero, and from this on the reaction is endothermic. It is 



hence evident that the reaction runs exothermically from either end 



p 

 down to the same point, where C = RT log ~, and at this point where 



H = 0, energy is neither given up to nor taken up from the surround- 

 ings. This point is accordingly the equilibrium point of the reaction. 



The equation for equilibrium is accordingly RT log p^ = C = const. 1 



If the chemical energy involved in the passage from the form A 

 to the form B, or vice versa, is so small as to be negligible, as is 

 usually the case in the passage of a substance from one isomeric form 



P PB 



to another, we can write RT log ~ = C = 0, and hence log p- = 0, 



-CA r l 



^ = 1, or P B = P A . That is, under such conditions equilibrium is 



attained when the osmotic pressures, and hence the molecular concen- 

 trations in solution, are equal. 



If we write p A and p^ for the osmotic pressures of the two sub- 

 stances at the equilibrium point, and P A and P B as before for the 

 corresponding pressures at any given point in the reaction, another 

 form can be given to the fundamental equation for the heat of 

 reaction at any given point in the reaction. 



For now C = RT log , and hence on substituting this value we 



PA. 

 obtain 



p P \ p P 



"PA g P A ,T >g ^I'lY 



2. Let us take next the cases where two substances A and B 

 interact to form reversibly two other substances C and D, and 

 let P, with the appropriate suffix of the letter denoting the sub- 

 stance, represent the osmotic pressure of each substance. 



Then since A and B disappear from solution and diminish the 

 volume energy or osmotic pressure energy, and C and D appear and 

 increase the osmotic pressure energy, the equation becomes 



H = C-RT (log ^ + log jr-log p A -log jr), 

 or 



P C 



1 This equation may also be written log -^ =^ 



Jr -tl . 



OP 

 or ~ e KT< 



